1. Field of the Invention
The present invention relates to an image forming apparatus for applying an alternating voltage superimposed on a direct current voltage to a developer bearing member to thereby develop an electrostatic latent image formed on an electrostatic latent image bearing member with a toner.
2. Description of the Related Art
In an electrophotographic image forming apparatus, a development method has been employed in which a surface of an electrostatic latent image bearing member (for example, a photoreceptor) is charged and an image is exposed to the charged region to from an electrostatic latent image, and the electrostatic latent image is developed so as to be made visible (developing).
As such a development method, a development method has been commonly used in which, using one-component developer containing a toner or two-component developer containing a carrier and a toner, by frictionally charging the toner so that the toner is attracted with an electrostatic force of an electrostatic latent image on the surface of the electrostatic latent image bearing member, the electrostatic latent image is developed to thereby form a toner image.
For example, when two-component developer is used, a method has been employed, in which a magnetic brush by carrier is formed on a developer bearing member (for example, a developing roller) in a developing device, and an electrostatic latent image is developed while applying a bias voltage between the developer bearing member and an electrostatic latent image bearing member.
Moreover, whether one-component or two-component developer is used, there is a case where development is performed using a toner that is charged with a polarity opposite to a surface potential of the charged electrostatic latent image bearing member, or a case where reversal development is performed using a toner that is charged with a polarity the same as the surface potential of the charged electrostatic latent image bearing member.
In addition, there is also a case where an electrostatic latent image that is formed on the electrostatic latent image bearing member is developed with the toner by applying an oscillating bias voltage between the developer bearing member and the electrostatic latent image bearing member. In this oscillating bias voltage, a development-side electrical potential, i.e., a for-development electrical potential, that can apply a force to the charged toner in the direction from the developer bearing member toward the electrostatic latent image bearing member and an opposite development-side electrical potential, i.e., an against-development electrical potential, that can apply a force to the toner in the direction from the electrostatic latent image bearing member to the developer bearing member alternate with each other, and for example, as shown in FIG. 9, a rectangular wave is commonly used whose ratio (duty ratio) of the application time during which the development-side electrical potential is applied to the application time of a cycle during which the development-side electrical potential and the opposite development-side electrical potential are applied is 50%.
Incidentally, in such a conventional development method, it is desirable that the charge amount of the toner is increased to obtain smooth image quality with little roughness. However, for example, when two-component developer is used, the electrostatic force between a carrier and a toner is in proportion to the square of the charge amount, thus, when the charge amount of the toner is increased, a rate that the carrier separates from the toner decreases. Accordingly, the utilization efficiency of the toner consequently deteriorates and the image density is reduced. In order to increase the image density, an oscillation amplitude voltage Vpp (peak-to-peak voltage) of the oscillating bias voltage may be increased. However, when Vpp is increased, an electric field in the direction where the toner returns from the electrostatic latent image bearing member to the developer bearing member is strengthened, thus a toner image that has been attached to the electrostatic latent image bearing member once is peeled off and dot is not added completely. That is, so-called dot reproducibility tends to deteriorate.
Therefore, in recent years, a configuration has been proposed that, to act an electric field with an AC electric field superimposed on a DC electric field in a developing area in which the developer bearing member and the image bearing member are opposed, development is performed by applying a developing bias voltage so as to alternately repeat a first period during which an AC voltage is acted between the developer bearing member and the image bearing member and a second period during which no AC voltage is applied, for example as shown in FIG. 10 (refer to, for example, Japanese Unexamined Patent Publication JP-A 7-311497 (1995)).
In addition, as shown in FIG. 11, a configuration has been also proposed that development is performed by slightly giving vibration at a high frequency in the second period during which no AC voltage is applied (refer to, for example, JP-A 11-44985 (1999)).
In an image forming apparatus described in the JP-A 7-311497, there is an effect that dot reproducibility is improved and unevenness in a halftone area is reduced to form a smooth image, however, a force of returning a toner from the electrostatic latent image bearing member to the developer bearing member is so weak that adhesion of the toner to a non-image area, so-called fog, is increased.
Similarly in a developing device described in the JP-A 11-44985, there is an effect that dot reproducibility is improved and unevenness in a halftone area is reduced to form a smooth image, however, a force of returning a toner from the electrostatic latent image bearing member to the developer bearing member is insufficient. An electric field is applied in a direction to return the toner from the electrostatic latent image bearing member to the developer bearing member as vibration is given in the second period, however, it is impossible to return the toner sufficiently due to a high frequency, thus increasing fog as well.